Nucleation rate analysis of methane hydrate from molecular dynamics simulations.

Daisuke Yuhara,Amadeu K Sum,Brian C Barnes,Kenji Yasuoka,Gregg T Beckham,David T Wu,Brandon C Knott,Donguk Suh

doi:10.1039/c4fd00219a

Abstract

Clathrate hydrates are solid crystalline structures most commonly formed from solutions that have nucleated to form a mixed solid composed of water and gas. Understanding the mechanism of clathrate hydrate nucleation is essential to grasp the fundamental chemistry of these complex structures and their applications. Molecular dynamics (MD) simulation is an ideal method to study nucleation at the molecular level because the size of the critical nucleus and formation rate occur on the nano scale. Various analysis methods for nucleation have been developed through MD to analyze nucleation. In particular, the mean first-passage time (MFPT) and survival probability (SP) methods have proven to be effective in procuring the nucleation rate and critical nucleus size for monatomic systems. This study assesses the MFPT and SP methods, previously used for monatomic systems, when applied to analyzing clathrate hydrate nucleation. Because clathrate hydrate nucleation is relatively difficult to observe in MD simulations (due to its high free energy barrier), these methods have yet to be applied to clathrate hydrate systems. In this study, we have analyzed the nucleation rate and critical nucleus size of methane hydrate using MFPT and SP methods from data generated by MD simulations at 255 K and 50 MPa. MFPT was modified for clathrate hydrate from the original version by adding the maximum likelihood estimate and growth effect term. The nucleation rates calculated by MFPT and SP methods are within 5%, and the critical nucleus size estimated by the MFPT method was 50% higher, than values obtained through other more rigorous but computationally expensive estimates. These methods can also be extended to the analysis of other clathrate hydrates.

Highlights

PaperClathrate hydrates are ice-like structures in which guest molecules are trapped inside water cages connected by a hydrogen-bonded network.[1]
We analyze the nucleation rate from this set using the mean first-passage time (MFPT) and survival probability (SP), and the critical nucleus size is calculated from MFPT
The nucleation rates obtained by MFPT and SP are in good agreement and these results are close to the rate Barnes et al estimated using direct calculations based on the maximum likelihood estimate

Summary

Introduction

Clathrate hydrates are ice-like structures in which guest molecules are trapped inside water cages connected by a hydrogen-bonded network.[1] The formation of clathrate hydrates of natural gases, called gas hydrates, is a serious problem in the ow assurance of oil/gas ow lines. Understanding hydrate formation is required to develop efficient energy production strategies. At the most fundamental level, the mechanism of hydrate formation must be understood, since the incipient hydrate crystallization phenomenon can be controlled in ow lines during the production of oil/gas and for the gathering of gas from hydrate reservoirs. A thorough understanding of the hydrate formation process will improve the efficiency of other challenging hydrate applications such as gas transport,[2,3] hydrogen storage,[5] and capture/sequestration of carbon dioxide.[6]

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